A principal value of cis-pinane lies in the synthesis of linalool. The reaction sequence from .alpha.-pinene to linalool is illustrated in the following equation. ##STR1##
The reaction sequence to cis-pinanol is disclosed in Bull. Soc. Chem., Vol. 10 (No. 657), pp. 4141-4145 (1968), the subject matter of which is incorporated by reference herein. The pyrolysis to linalool may be found in U.S. Pat. No. 3,240,821, Ohloff et al, also incorporated by reference herein. Preferably, the hydrogenation of .alpha.-pinene to pinane is carried out under pressure over a nickel catalyst, and the product mixture, after separation from the catalyst, is oxidized with elemental oxygen in the presence of azobisisobutyronitrile to produce pinane hydroperoxide. This product mixture is then subjected to reduction in the presence of hydrogen and a catalyst to reduce the hydroperoxide to pinanol which is pyrolyzed to linalool.
In the above oxidation and reduction steps, there is formed some trans-pinane hydroperoxide and trans-pinanol in the ratio of about 80 cis-:20 trans-. For the sake of simplicity, only the predominant cis- forms are shown. In the pyrolysis step, both the cis- and trans-pinanol are pyrolyzed to linalool.
A primary problem in the reaction sequence is that, in the oxidation to the hydroperoxide, the cis-pinane oxidation occurs at a much faster rate than the trans-pinane oxidation, on the order of about 10:1. (Reported by Fisher et al* and confirmed by Applicant' s observations.) The oxidation could be allowed to run to substantial completion and oxidation of trans-pinane, but during the oxidation step, decomposition of the hydroperoxide product occurs which increases with increased concentration of the hydroperoxide and by-products of the oxidation reaction. This necessitates stopping the reaction as a practical matter at about 50% conversion of the pinane. Distillation of the product mixture is then carried out and unreacted pinane is recycled for addition to fresh pinane and further oxidation. FNT *JACS, 75- 3675-3678 (1953).
Fortunately, the initial hydrogenation step is stereoselective in favor of cis-piane so that the recycled unreacted pinane mixture from the oxidation step contains only a small amount of the less-readily oxidizable trans-pinane. Even so, it is apparent that after multiple recycles; e.g., 5 or 6, the recycle will contain a sufficiently high level of trans-pinane to require its disposal or purge. This purge is accomplished by accumulating the high-trans containing recycle material over a substantial period of time and then subjecting the same to successive oxidation and reduction steps, without addition of fresh pinane, each time producing additional hydroperoxide and consuming or reducing cis-pinane content, until the cis-pinane:trans-pinane ratio reduces to about 50:50.
At this point reoxidation of the pinane mixture is terminated, and the pinane is disposed of with an obvious purge of still unreacted cis-pinane. The cis- and trans-pinanes are inseparable by fractional distillation, so that this represents a substantial loss of desired cis-pinane.
The hydrogenation of .alpha.-pinene to cis- and trans-pinane has heretofore been carried out using a nickel hydrogenation catalyst, such as a supported catalyst Girdler G-69 (trademark, Girdler Chemicals, Inc.); or a Raney nickel catalyst (trademark, W. R. Grace & Co.) which is unsupported. Also, a partly colloidal Rufert nickel catalyst (for instance, Ni-5000 F, Harshaw Chemical Company, trademark) can be employed. The present invention is applicable to any nickel catalyst conventionally used in the hydrogenation of olefinic bonds, although it will be apparent that the present invention is primarily applicable to the use of the supported and Raney nickel catalysts.
In the conventional hydrogenation of .alpha.-pinene, selectivity to cis-pinane is experienced, which varies depending upon temperature and reaction time, the selectivity being better at lower temperature and longer reaction time. In this regard, Applicant's experience has been that a typical cis:-trans-ratio will vary from about 87:13 (for about 2 hours' reaction time) to a maximum of about 95:5 for longer reaction times, for instance about 14 hours. Up to 30 or more hours' reaction time is reasonable. Use of a Rufert catalyst may result in increased cis-:trans-ratios, but the use of this catalyst has the disadvantage that it cannot be separated from the reaction products by simple filtration, requiring distillation or clay treatment for complete removal of nickel from the reaction mixture.* The removal of the nickel is necessary as it tends to cause acceleration of the decomposition of the pinane hydroperoxide to undesirable products during the oxidation step. FNT * Catalysis, Vol. III, page 426, Paul H. Emmett, Reinhold Publishing Corporation, 1955.